This invention relates generally to systems for decoding television signals having a second audio program (SAP) and stereo audio signals.
Recently the Federal Communications Commission authorized broadcast transmission of television (TV) signals bearing coded stereo audio information, SAP information, and other information, generally useful to broadcast/cable operators. A TV must be specially designed to receive and detect these audio signals. Alternatively, a separate decoder may be used in conjunction with a conventional TV since all of the added information is included with the conventional audio carrier. The TV decoder, or separate decoder must determine which of the signals are present and reproduce the audio signal that the viewer selects. The SAP information, which may comprise a foreign language accompaniment, is a preferred mode of receiver operation for many viewers and is therefore the source of choice when it is available.
Prior art decoder systems include a microprocessorbased selection switch that is viewer operated for choosing between the SAP and stereo audio formats. A detection circuit that is responsive to the SAP carrier (equal to five times the TV horizontal line frequency) generates a DC signal indicative of the SAP carrier level. When SAP information is not being telecast, as evidenced by lack of a SAP carrier, the microprocessor causes the decoder to shift to the stereo audio mode. The prior art SAP decoder is expensive and prone to erroneous operation, especially when no SAP information is present or when the SAP carrier is weak. Consequently, there is a need in the art for more reliably determining the presence of SAP information for controlling the automatic switching from SAP to stereo.